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Glad you have solved the mystery of your tuner's behavior!
Here's an article that you -- and others -- might enjoy:
"To achieve the highest possible efficiency at a given impedance transformation, tune the network with the highest output capacitance that allows a match."
The section on the last page, "For Tapped-Inductor Tuners" summarizes the process to follow to accomplish the above.
On Jun 6, 2020, at 5:19 PM, Dave W0DCX <cwqrp73@...
Well well well. This has been pretty interesting, somewhat humiliating, and rather humorous. I followed Jim's directive and, at least with the dummy load in place, the Tuner at last seems to be behaving as one might expect. So maybe I did assemble it correctly! Who would have thought! In the final step, as per Jim's advice, I replaced my NanoVNA with my transceiver set on 40-meters at 1 watt and obtained the same LED pattern on the Tuner. Interestingly, when I switched to 20-meters at first it seemed not to work well. But I was assuming there was an incremental pattern of tuning to be expected when adjusting the inductor. I had previously assumed that I could "bracket" the inductor settings to more quickly figure out the best tuning, but I guess not. I think one has to try each and every inductor setting more methodically. Lo and behold I found that 20-meters gave me the green light when I had the inductor at the B position, Cap-L at 10 and Cap-R at 0. B is the next to highest setting of A which worked well for 40-meters. Lesson learned is that I need to be completely empirical when trying to adjust the tuner for the best match and not assume any patterns will emerge. But the big question now is how will this all work with an actual antenna instead of a dummy load? I was flummoxed when I first tried it on my AX1 vertical whip, but maybe now that I understand the tuning process better I can get it to work with that and other antennas too.